A typical power generation plant includes a gas turbine having an axial compressor that feeds compressed air to a plurality of combustors. The compressed air is mixed with a fuel to form a combustible mixture within each combustor. The mixture is burned, thereby producing a rapidly expanding hot gas. The hot gas is routed through a hot gas path and into an expansion turbine.
As the hot gas flows through the turbine, kinetic energy is transferred to one or more rows or stages of turbine blades which are coupled to a shaft that extends axially through the gas turbine, thereby causing the shaft to rotate. The hot gas is exhausted from the gas turbine through an exhaust gas diffuser. The shaft drives the compressor to sustain operation of the gas turbine. A generator/electric motor is coupled to the shaft upstream from an inlet of the compressor. The generator converts the rotational movement of the shaft into electricity. The generator/electric motor drives the compressor during start-up of the gas turbine.
Because the gas turbine is an air breathing engine, output of the gas turbine is affected by anything that affects the density of the air entering the compressor. For example, power plant site conditions such as ambient air temperature, barometric pressure (i.e. site elevation) and relative humidity contribute to overall gas turbine output. Gas turbines are generally designed to operate at a reference temperature of approximately 59 degrees Fahrenheit, an atmospheric pressure of approximately 14.7 psia and a relative humidity of approximately 60%. However, few power plant sites realize these reference conditions during an entire day or throughout the year. In addition, most sites require that air filtration equipment, silencing devices, evaporative coolers or chillers be placed at the inlet of the compressor, thereby causing pressure losses within the gas turbine system. As a result, power output of the gas turbine is decreased.
Various systems have been employed to augment/optimize power plant efficiency in order to accommodate for non-reference temperature and pressure operating conditions. For example, one such system injects compressed air from a compressed air storage system, such as an underground cavern or other containment vessel, into the combustion section of the gas turbine, particularly during peak operation/demand. Generally, the compressed air storage system is charged or pressurized with compressed air from a secondary compressor driven by a motor during off peak periods of operation. One issue with this system is the additional costs associated with installing and maintaining a separate compression system to pressurize the compressed air storage system.
Although overall power plant output is increased and overall heat rate is decreased by utilizing compressed air injection into the combustion section of the gas turbine, operators continue to seek new ways to further increase power plant output while at the same time reducing costs. Therefore, an improved system and method for augmenting or improving overall power plant efficiency would be useful.